1. Field of the Invention
The invention relates to a system and a method for distributing positional information and location specific messages to mobile units by using a data network.
2. Description of the Related Art
Global positioning system (“GPS”) is used for obtaining position information. The GPS includes a plurality of satellites orbiting the earth and sending out, at precisely synchronized times, a code sequence that identifies the satellite. Specifically, the code sequence transmitted by each satellite is a precisely timed binary pulse train. In addition to the code sequence, these satellites send out positioning information that can be used to calculate vehicle positions. A GPS receiver retrieves the positioning information from the GPS satellites and triangulates the information to obtain the position of the receiver. A more detailed discussion of a GPS receiver is found in U.S. Pat. No. 5,990,827 to Rodric C. Fan, et al. entitled “Structure of a Position Processing Apparatus,” which is herein incorporated in its entirety.
One application of GPS is vehicle location. A conventional vehicle locating system typically includes one or more ground stations and a plurality of mobile units that may be installed in vehicles. Each mobile unit is equipped with a GPS receiver and a wireless transmitter. Using the GPS receiver, a mobile unit determines the position of the vehicle in which it is installed and transmits the position to a ground station. The ground station of a conventional vehicle locating system normally includes a map database search system and some type of storage medium that stores digital maps and travel-related information. Thus, upon receiving the vehicle positions from the mobile units, the ground station combines the stored data with the received information and displays the present vehicle positions on a digital map.
In some cases, a data network, such as the Internet, is involved in locating mobile units. When a data network is involved, the mobile units determine the vehicle positions and transmit the position information to the network server. This data transmission from a mobile unit to the network server is accomplished wirelessly by, for example, cellular digital packet data network (CDPD) that connects to a data network (e.g., the Internet), which in turn provides access to the network server. In some cases, the mobile units may transmit raw data to the network server so that the network server can make the position determination. In a system that involves a network server, the data storage medium may be accessed by the network server instead of the ground station. The data storage medium contains travel-related information such as maps, traffic conditions, positions of service stations, and destinations of interest. Thus, the data processing unit organizes the measured position and generates an area map. The area map indicates by a position marker the position of each mobile unit.
As mentioned above, vehicles can connect to data networks (e.g., the Internet) wirelessly through communication networks, for example a CDPD network. CDPD networks consist of Mobile Data Intermediate Systems (MDISs), Mobile Data Base Stations (MDBSs), and Mobile End Stations (MESs). An MDBS offering CDPD services helps a roaming MES register with the MDIS with which the particular MDBS is associated by acting as a conduit for the registration message. An MDIS that handles the registration is referred to as the serving MDIS. Upon receiving the registration message from the MDBS, the serving MDIS informs the home MDIS of the MES of the latter's presence in its coverage area. When a host needs to send data to an MES, it does not have to be aware of the mobility aspect of the MES; it simply transmits data using the MES's IP address as the destination address. The encapsulated data packets for the MES are forwarded to the serving MDIS of the MES. At the serving MDIS, packets are decapsulated to reveal the MES's address. The serving MDIS sends the original data packets to the right channel where the MES is currently located. The MES receives the data packets. If the MES needs to reply, it directly sends data packets using the remote host's IP address as the destination address. CDPD systems are further described in K. Budka et al., “Cellular Digital Packet Data Networks,” Bell Labs Technical Journal, Vol. 2, No. 3 (Summer 1997); “Cellular Digital Packet Data Systems Specification: Relases 1.1,” CDPD Forum, Inc., Chicago (1995); and M. S. Taylor et al., “Internet Mobility: The CDPD Approach,” Prentice Hall PTR, Upper Saddle River, N.J. (1996).
The vehicle locating system described above, while useful for tracking the fleet of a trucking company or a moving company, is limited in that the positions of the vehicles can only be obtained from a ground station. A member of the fleet cannot use the above vehicle locating system to obtain vehicle position information of other fleet members. A method and system that allows one member of the fleet to obtain vehicle position information of other members of the fleet from a place other than the ground station is needed.